1. Field of the Invention
This invention relates to a semiconductor integrated circuit.
2. Related Background Art
In general, Semiconductor integrated circuits having a power transistor suffer from heat generated by electric current flowing through the power transistor. The heat may raise the temperature of the junction of the power transistor, and thereby may break the power transistor and elements placed around it.
As a countermeasure, an overheat detector circuit has been provided heretofore to detect the temperature of the junction of the power transistor.
FIG. 8 is a schematic diagram of a conventional semiconductor integrated circuit 800 having an overheat detector circuit. The semiconductor integrated circuit 800 includes a bipolar transistor 10, driver 20 for driving the transistor 10, reference voltage circuit 30 for generating a reference voltage, resistor R1 connected between a power source and an emitter, an over-current detector circuit 50 which detects the emitter current by comparing a comparative voltage from the power source via the resistor R1 with the reference voltage and thereby controls the driver 20, and an overheat detector circuit 65 for controlling the driver 20 in accordance with the temperature of the junction of the transistor 10.
Operation of the semiconductor integrated circuit 800 is explained below with reference to FIGS. 8 and 9. FIG. 9 shows a graph of changes in the quantity of emitter current IE with temperature Tj at the junction of the transistor 10.
Since the comparative voltage is applied from the power source VCC through the resistor R1, it varies with the emitter current IE. The reference voltage is a constant voltage determined by the electromotive force of the reference voltage circuit 30.
The over-current detector circuit 50 compares the comparative voltage with the reference voltage, and controls the driver 20 to prevent the emitter current IE from increasing beyond a predetermined value Ioc (see FIG. 9).
If the emitter current Ioc continuously flows in the transistor 10, the junction temperature Tj of the transistor 10 rises because the energy VCE*Ioc continuously generates at the junction of the transistor 10, where VCE is the collector-emitter voltage. Continuous rise of the junction temperature Tj will break the transistor 10. Therefore, the overheat protective circuit 65 controls the driver 20 to prevent the junction temperature Tj from surpassing a predetermined value Tot.
That is, in FIG. 9, when the junction temperature Tj reaches Tot, the driver 20 shuts the transistor 10 off so as not to move it into the state of the shadowed region Sb. As a result, the transistor 10 turns OFF and the emitter current IE stops.
In this manner, the overheat protective circuit 65 was heretofore used to protect the transistor 10 and its peripheral elements from destruction by overheating.
However, in case the rising rate (for example, K/s (Kelvin per second)) of the junction temperature Tj per unit time is too high for the overheat detector circuit 65 to catch up to for detection, or in case the junction temperature Tj is already high when the transistor 10 is activated, the transistor 10 may undesirably move into the state of the shadowed region Sb.
For example, assume here that the junction temperature Tj is high when the transistor 10 is activated and that the transistor 10 is in the state of the region Sa of FIG. 9. Even in this case, the over-current detector circuit 50 permits the emitter current Ioc to flow into the transistor 10. Therefore, the transistor l heretofore taking the state of the region Sa shifts to the state of the region Sb before the overheat detector circuit 65 can detect it and turns OFF the transistor 10.
Therefore, the semiconductor integrated circuit 800 even with the overheat detector circuit 65 was still unable to reliably protect internal elements from heat of the power transistor.
It is therefore desirable to provide a semiconductor integrated circuit which is capable of reliably protecting the power transistor and its peripheral elements against heat of the power transistor.